In general, a pneumatic tyre comprises an internal sealing layer delimiting the internal volume of the pneumatic tyre. This layer generally comprises a rubber of the butyl type which is known for its airtightness.
In use, the pneumatic tyre may become punctured if a perforating object, for example a nail, perforates the pneumatic tyre. Such perforation causes the pneumatic tyre to lose air.
In order to prevent the pneumatic tyre from losing air, it has been proposed that an additional layer of a relatively soft product capable of creeping readily be positioned in contact with the internal sealing layer. Thus, in the event of a puncture, the product of the additional layer, because of its softness and its ability to creep readily, penetrates the puncture and prevents the pneumatic tyre from losing air. Such a product with relative softness and the ability to creep readily is said to be self-sealing.
However, developing a self-sealing product is a relatively complex process. Specifically, if the product is too soft and/or has too great an ability to creep, the self-sealing product may creep under the effect of centrifugal force when the pneumatic tyre is being used. Thus, the product creeps axially towards the centre of the pneumatic tyre. The axially external parts, or shoulders, of the pneumatic tyre are then not as well protected. Furthermore, the product may creep axially towards the centre of the pneumatic tyre even when the pneumatic tyre is stationary, particularly under conditions of high temperature. In addition, if the puncturing object is removed from the pneumatic tyre, the self-sealing product may creep out through the puncture and escape from the pneumatic tyre. The function of preventing air loss is then no longer assured. Finally, the self-sealing product may interact chemically with intermediate storage inserts. When the layer of product is assembled onto the green form of the pneumatic tyre on a tyre-building drum, the self-sealing product may also interact with the tyre-building drum. In particular, it may also interact by sticking to the curing membrane of the vulcanizing press, soiling it and leading to the need to shut down the vulcanizing plant.
Furthermore, if the product is too rigid and/or does not have a great enough ability to creep, the product does not creep into the puncture sufficiently, particularly in cold weather.